1. Field of the Invention
The present invention relates to an intake air temperature sensor, and particularly to an intake air temperature sensor with little effect of the temperature of a sensor fixing member and self-heating of a circuit section, and a thermal airflow meter including the same.
2. Background Art
For instance, JP Patent Application Publication No. 2005-9965 A (2005) discloses an intake air temperature sensor in which an intake air temperature is detected based on information of a first temperature sensor disposed at a flow rate measuring element and a second temperature sensor disposed in a casing.
However, the technique described in JP Patent Application Publication No. 2005-9965 A (2005) does not give much consideration to air temperature variation in a secondary passage due to the temperature of a secondary passage structure, an effect on the second temperature sensor due to self-heating of circuits disposed in the casing, the thermal time constant of the casing from several tens of seconds to several minutes and the like.
The structure of the secondary passage has a temperature different from that of an airflow to be measured owing to heat from a sensor fixing member and self-heating of circuits. Accordingly, owing to adverse effects of heat from the structure of the secondary passage, the temperature of the air in the secondary passage is changed and becomes a temperature different from that of the airflow to be measured, which causes a problem in that an error occurs in an output of the intake air temperature sensor in the above technique.
Further, since variation in the temperature of the structure of the secondary passage is affected by the thermal time constant of the casing, several tens of seconds to several minutes are required to stabilize the temperature of the structure of the secondary passage. Accordingly, the above technique has a problem that the output of the intake air temperature sensor is unstable for several tens of seconds to several minutes after the intake air temperature is changed.
Variation in airflow rate varies the power consumption of a drive circuit for driving an airflow rate sensor and in turn varies the self-heating of the circuits. Here, time until the temperature of the casing is stabilized is also affected by the thermal time constant of the casing. Accordingly, it is required several tens of seconds to several minutes until an output from the second temperature sensor is stabilized. This causes a problem in that the output of the intake air temperature sensor, which is corrected on the basis of the output of the second temperature sensor, is also unstable for several tens of seconds to several minutes after the airflow rate varies.